fixed OSError message and added file path validation

[yugui923]

Hi Belval - thanks for writing and maintaining the pdf2image package!

I created this pull request to correct a miscategorized error message and implement more robust validation logic for inputs. Specifically:

1. "pdf_path" argument was not validated in the "pdfinfo_from_path" function
2. Further to the lack of validation, the entire "pdfinfo_from_path" function is covered by a try-except block, which catches all OSError and then displays a PDFInfoNotInstalledError (see below).

 except OSError:
        raise PDFInfoNotInstalledError(
            "Unable to get page count. Is poppler installed and in PATH?"
        )

3. The combination of the above two factors will lead to wrong error messages being displayed when the "pdf_path" is invalid. Instead of raising an OSError and alert the user that the "pdf_path" is invalid, the current program will display a PDFInfoNotInstalledError, and falsely alert the user to check their poppler installation.

I've added a very simple fix to this error message and added validation logic in this PR

Error message for invlid path after fix:

  File "d:\vc-memo-generator\mg-backend\.venv\Lib\site-packages\pdf2image\pdf2image.py", line 579, in pdfinfo_from_path
    raise OSError(f"PDF file not found: {pdf_path}")
OSError: PDF file not found: /peloton_presentation_may_2024.pdf

Error message for invlid path prior to fix:

  File "d:\vc-memo-generator\mg-backend\.venv\Lib\site-packages\pdf2image\pdf2image.py", line 631, in pdfinfo_from_path
    raise PDFInfoNotInstalledError(
pdf2image.exceptions.PDFInfoNotInstalledError: Unable to get page count. Is poppler installed and in PATH?